The invention relates to an optical scanning device comprising an objective system, a rotationally symmetrically rotating reflector body comprising a plurality of reflector elements each comprising two plane reflector faces which are disposed at an angle of substantially 90.degree. to one another and which intersect each other along a line transverse to the direction of rotation of the reflector body, a radiation-sensitive detection system arranged in the path of the radiation reflected by the reflector body, and an imaging system arranged between the detection system and the reflector body for imaging a point of the detection system and a fixed point in the proximity of the reflector body onto one another.
The imaging system may comprise one lens element or a plurality of lens elements and is also referred to as "relay lens". The point in which a point of the detection system is imaged by the imaging system may be situated inside a reflector element but also outside such an element in the vicinity of the reflector body. These possibilities are referred by the expression: "in the proximity of the reflector body". The detection system may comprise a single detector but alternatively a plurality of detectors arranged in line.
Such a device is disclosed in British Patent Specification No. 1,287,280. The known device comprises a rotating disc which at its circumference carries a plurality of roof mirrors or facet mirrors each comprising two reflecting faces disposed at an angle of 90.degree. to one another. The scene to be scanned or the object to be scanned is imaged in the proximity of the disc by the objective system and the roof mirrors reflect the radiation of the scanning beam to a radiation-sensitive detection system comprising a single detector.
The operation of scanning devices of this type is generally described starting from the detection system and following the radiation path from this system back into the space to be scanned or object space. This method will also be adopted in the following description. In the reverse radiation path between the reflector disc and the detector a lens is arranged which images the detector in a fixed point of a curved surface through the apexes of the roof mirrors. During rotation of the reflector disc a scene is scanned along a line, for example, a horizontal line. Scanning in a second direction, for example the vertical direction, can be effected by means of a plurality of detectors arranged along a line parallel to the axis of rotation of the reflector disc.
In comparison with other scanning bodies, such as a rotating transparent prism or a drum whose outer circumference carries a plurality of single mirors, the use of a disc with roof mirrors, also referred to as facet disc, in a scanning device has the advantage that this device does not have a "dead time", i.e. there is no time interval between the instant at which scanning along a first line terminates and the instant at which scanning along a second line begins. When this scanning device is employed in a thermal imaging or infrared camera, in which a cold shield is arranged before the detector, another advantage of the facet disc is that the beam of useful radiation reaching the detector is stationary in space, so that the aperture in the cold shield can be small, which ensures that the background radiation which is incident on the detector is minimal.
As the imaginary beam issuing from the detector is focussed in a point near the reflector faces, this beam will have a small cross-sectional area at the location where it is incident on a first face of reflector element. Conversely, the scanning beam issuing from the scene or the object which is incident on the first face via the second face of the same reflector element will have an equally small cross-sectional area. This means that small disturbances in surface of the first reflector face, in the form of scratches or pits, or dust particles on this surface, will have a comparatively large influence on the radiation intensity of the scanning beam and hence on the signal supplied by the detector. When the scanning device is employed in an infrared camera said disturbances not only reduce the amount of useful infrared radiation incident on the detector but these disturbances themselves also emit infrared radiation to the detector. In the visible image of the infrared scene these disturbances may appear as specks, in particular if the actual picture is uniform.